Most conventional sports rackets have strings which are effectively anchored as they pass through string holes and about the frame of the racket. The isolation of each string chords causes localized high dynamic string tension to be developed in the chords which are actually struck by a ball or other object. The dynamic string tension, static string tension, string elasticity, frame stiffness, strung area geometry, and racket balance are all factors known to affect power, control, comfort and equipment performance. Many racket shapes, sizes, materials, string tensions, weights, even grips have been tried to improve racket performance.
U.S. Pat. No. 3,999,756 to Head teaches an enlarged string area for tennis rackets to enhance physical properties such as the location of the center of percussion and the magnitude of the polar moment of inertia. The Head racket is shown with each string chord being a different length than its neighbor. Each string chord is effectively anchored at opposite ends as it bends over a resilient pad on the frame and extends to the next adjacent hole. The anchored string relationship causes high dynamic tension along individual string chords during impact with resulting problems of string and racket breakage, sensitivity to weather, and wear.
U.S. Pat. No. 1,559,986 to Quick and U.S. Pat. No. 4,462,592 to Legger et al. exemplify spring devices for mounting tennis racket strings. They make use of various pulley or guide arrangements mounting the string chords with springs. The springs function as reaction members with responses varying depending upon the spring's resistance to compression or extension. These patents are also illustrative of slides or sheave arrangements guiding the string courses across the racket face. While sheaves have some effect on transmission of tension from one string chord to the next, the transmission of tension is limited by the resilient action of the spring which tends to minimize tension.
The 1942 French patent to Lemoine discloses another form of spring mechanism between adjacent string chords in a tennis racket head. In one form, the spring is formed of a wire band for yieldably receiving the string. In another form, a wire spring-action band mounts a pulley or sheave for receiving the string. In yet another form, Lemoine uses both a spring band and an elastic material partially enclosed by the band. In the later form, the springs are positioned within recesses in the racket head frame with ends of the springs abutting walls of the recesses. Rocking movement of the springs is prevented by the walls of the recesses. The Lemoine spring members do not rock in response to differential tension in adjacent string chords. Instead, the Lemoine devices respond to the force existing in one chord and limit the tension by resilient deformation of the spring member.
Further spring mechanisms mounted between adjacent string chords on a tennis racket head are disclosed in British Patent309,238 granted in 1928 to Brooks and in U.S. Pat. No. 1,523,865 to Craig. These patents disclose several spring configurations by which tennis racket string chords may be resiliently mounted to a racket head. The springs do not rock in any manner to transmit tension, but instead deflect in response to applied tension.
A 1914 British patent to Lidy discloses a tennis racket having a string tension adjusting mechanism. The arrangement incorporates a hollow head frame in which a series of substantially spherical rollers are situated to receive the various string chords. The rollers are claimed to even tension over the racket surface during tension adjusting operations. However, rollers respond to uneven string tension by rolling along the racket head in the direction of the tension. This results in mislocated strings.
U.S. Pat. No. 4,441,712 to Horst Guthke illustrates a tennis racket string mount. Strings are connected to the racket frame through a number of linking elements that permit displacement of the string connection points relative to the racket head frame. A complex linkage and pin arrangement is used to eliminate asymmetrical deflection of the strings in the area adjacent the head frame. The linkages are used to essentially change the anchor point of the various strings in relation to the racket head frame. The result claimed is establishment of the advantages of a large racket head in a conventional size head frame. However, the string connections extend into the string opening of the racket shortening the string chord lengths. Reduced string length reduces the available dynamic strain or elongation which the string chords can produce. Furthermore, the various pivots reduce efficiency due to friction. The Guthke invention further includes pulleys or sheaves within the racket face which support a single strand of string arranged through the pulleys. The pulleys further extend into the string opening shortening the overall string length and decreasing the opening size. The pulley and link arrangements add significantly to the head weight of the racket. Other pulley arrangements are shown in U.S. Pat. No. 2,145,785 to Aubert, and in U.S. Pat. No. 4,203,597 to Reedhead.
There remains a need for optimizing power and control of a stringed racket while minimizing the complexity of mechanisms for achieving such results. It is also desirable to incorporate simple, effective, features in a stringed racket that will produce a higher coefficient of restitution thus providing increased power for the same exertion. Rackets which display lower stress in the racket frame during the hit are also desired because of the lower magnitude of vibrations experienced by the player. It is also desirable to lower the peak tension in the strings during the hit and to balance restoring forces. Restoring forces can be balanced by providing even dynamic string tension as seen by the hit object, preferably approaching that of a uniform membrane. Such uniformity reduces edge effects on off-center hits. It is additionally desirable to provide a longer dwell time for better control and larger string deflection for better energy transfer and additional power return to the ball.